FIG. 1 is an example of connection between the frequency convertor and prescaler of an existing superheterodyne receiver. FIG. 2 is a detail circuit of a part of the frequency convertor and prescaler of FIG. 1.
In these figures, 1 is a local oscillator, 2 is a buffer amplifier; 3 is a receiving signal input terminal; 4 is an input signal filter; 5 is a mixer; 6 is an intermediate signal output terminal; 7 is a prescaler; 8 is a divided frequency output terminal; 9 is an injection capacitor; 10 is a local oscillation signal injection circuit; 11 and 12 are voltage dividing resistors; 13 and 14 are coupling capacitors; 15 is a trap coil; 16 is a trap capacitor; 17 is a prescaler integrated circuit (IC).
A local oscillation signal generated by the local oscillator 1 is amplified by the buffer amplifier 2 and then sent to the mixer 5. Meanwhile, a receiving signal applied to the receiving signal input terminal 3 passes the input signal filter 4 and is then applied to the mixer 5. It is mixed therein with a local oscillation signal and is converted to an intermediate frequency and then extracted from the intermediate frequency signal output terminal 6. In addition, the local oscillation signal is partly divided by the prescaler 7 and is then extracted from the divided frequency output terminal 8.
Explanation will then be continued further in accordance with the detail circuit diagram of FIG. 2. An output of the buffer amplifier 2 is guided to the mixer 5 by the local oscillation signal injection circuit 10 and injection capacitor 9 and is then applied to the prescaler IC 17 from the local oscillation signal injection circuit 10, partly branching the local oscillation signal through the voltage dividing resistors 11, 12, coupling capacitors 13, 14, trap coil 15, trap capacitor 16. Thereby, a frequency divided output of said local oscillation signal can be extracted from the divided frequency output terminal 8.
Here, a voltage dividing resistor and trap circuit operate as follows. The prescaler IC 17 is ordinarily composed of the flip-flop circuit and the frequency dividing ratio is determined by the number of stages. Accordingly, the flip-flop circuits of many stages are required in order to obtain a large dividing ratio. The waveform of the frequency-divided signal is almost rectangular including many harmonics. Of those harmonics, the 3rd harmonic is most intensive. Namely, in the process that the frequency is divided into 1/2, 1/4, 1/8, . . . by the flip-flop circuits, the frequency components 3/2, 3/4, 3/8, . . . are generated and these harmonics are also applied to the mixer 5 passing the local oscillation signal injection circuit 10 from the input circuit of the prescaler. As a result, unwanted beat interference occurs.
The trap circuit shown in FIG. 2 is inserted for attenuating harmonics of the frequency divided by the prescaler and the voltage dividing resistors 11, 12 sets the local oscillation signal level to the prescaler IC 17 to the optimum condition and attenuates the interference signal sent from the prescaler IC 17.
According to an existing example, as explained above, many trap circuits being suitable for the frequencies of respective signals are required in order to perfectly attenuate many interference signals. But it is very uneconomical. Moreover, there is such a disadvantage that since the trap circuit attenuates not only the interference signal generated by the prescaler IC 17 but also an input signal in the same way if it has the same frequency as said interference signal, such an input signal to the prescaler IC 17 is also attenuated in the vicinity of the trap frequency when the local oscillation signal which can be varied in broad band is divided, resulting in erroneous frequency dividing operation. Further, aforementioned prior art is followed by a problem that it is very difficult to adjust an input level to the optimum level.